Reduced bone mineral density and impairments in linear growth are common in children with cystic fibrosis (CF). Although calcium is the major mineral of bone, the role of calcium metabolism in CF has not yet been explored. The secondary consequences of CF on bone development and calcium status may be especially detrimental in girls, who have a lower peak bone mass than males and whose growth may be most delayed during the pubertal growth spurt. To address these issues, stable isotopes of calcium will be used to examine calcium absorption and rates of bone calcium turnover in girls with CF who are between the ages of 7-18 years. Biochemical markers of bone turnover (osteocalcin and N-telopeptide), pubertal maturation (estradiol and tanner stage), growth factors (insulin-like growth factor-1), calcium metabolism (vitamin D and parathyriod hormone), inflammation (C-reactive protein) and bone density will also be assessed and correlated with the calcium kinetic measurements. Stable isotopic techniques allow for the precise measurement of calcium absorption, distribution of calcium within the body and rates of bone calcium deposition and resorption. These techniques will provide useful information to better understand the mechanisms behind the reduced bone mineral density found in children with CF. As the lifespan of individuals with CF increases, it becomes more important to prevent chronic conditions such as osetoporosis that are largely irreversible once established. Results from this study will provide valuable data that will in turn help physicians to suggest interventions to maximize calcium retention and peak bone mass in children with cystic fibrosis."